U.S. Pat. Nos. 5,100,585, 5,110,474 and 5,346,618, whose disclosures are incorporated by reference, teach the use of several crown ether compounds in extracting various ions from 1-6 molar nitric acid solutions under various extraction conditions, including dispersal of the crown ether on an inert resin substrate material. Of the several crown ethers discussed, 4,4'(5')-[di-t-butyldicyclohexano]-18-crown-6 (Dt-BuCH18C6) was shown to be particularly effective at extracting strontium ions from nitric acid solutions.
This crown ether, Dt-BuCH18C6 (Compound 2), is prepared by the catalytic hydrogenation of the corresponding 4,4'(5')-[di-t-butyldibenzo]-18-crown-6 (Compound 1), as is shown generally below in Scheme 1. ##STR1##
Several isomers can form during the hydrogenation reaction. Some of those isomers do not effectively coordinate (complex) with strontium ions (Sr.sup.+2), whereas one or more isomers coordinate with strontium ions very well. Coordination (complexation) between the crown ether and strontium ions is the first step in extraction of strontium ions from an aqueous medium into an organic medium containing the macrocyclic ether. Such extraction provides a useful measure of coordination or complexation ability of the crown ether.
Of the isomers of Compound 2 formed during the catalytic hydrogenation shown in Scheme 1, one isomer, believed to have both cyclohexane rings cis-fused in a syn-configuration (cis-syn-cis; Compound 2b) is an excellent extractant (coordinator) of Sr.sup.+2 ions, whereas isomers in which the cyclohexane rings are thought to be trans-fused to the crown ether ring (e.g., Compound 2a; trans-anti-trans) do not extract (coordinate with) strontium ions in a model extraction system. ##STR2##
In the above compound depictions, wavy lines are used to indicate that the depicted bond can be above or below the plane of the page. Darkened wedge-shaped lines are used to indicate bonds that extend above the plane of the page, whereas dashed wedge-shaped lines are used to indicate bonds that extend below the plane of the page.
Put in numerical terms, the distribution ratio, D, between an aqueous 1M nitric acid solution containing tracer amounts strontium ions and three volumes of n-octanol containing a trans-fused crown ether at 0.5M is zero, whereas the ratio for the cis-syn-cis isomer is about 5-6. These studies are discussed in part in U.S. Pat. No. 5,100,585.
Effective extraction of strontium ions from nitric acid solutions of dissolved waste sludge of nuclear reactors or other media does not require absolute purity of the crown ether. A distribution ratio, D-value, of 2 or more, and preferably about 3 or greater provides adequate separations.
Achieving an adequate quantity of the cis-syn-cis isomer of Compound 2; i.e., Compound 2b, in the crown ether has been costly and problematic. Thus, reduction levels of starting material, Compound 1, have been erratic and are often in the range of about 10 to about 65 percent, with some preparations having almost none of the desired isomer for no apparent reason. When the desired cis-syn-cis isomer is present, that material is typically separated by column chromatography on alumina, which is useful but adds to the cost of the extractant.
It would therefore be beneficial if a process were available for consistently preparing an enhanced yield of the desired Compound 2 that contained an enhanced amount of the desired cis-syn-cis isomer, Compound 2b. It would also be beneficial if such a process could produce a product with sufficient isomeric content of Compound 2b so that material could be utilized without purification except for separation from the catalyst. The present invention provides such processes.